Solder coatings are widely used to protect the conductive substrate (typically copper) of printed wiring board (PWB) circuitry and electronic component leads from oxidation that causes loss of solderability. Solder coatings usually comprise eutectic Sn-Pb solder applied either by hot dipping or electroplating. Electroplated solder coatings are generally densified by subsequent reflowing (melting). Numerous studies and practical experience have shown that solder coatings, when properly applied and sufficiently thick, remain solderable even after several years of normal storage. In the molten state, however, solder tends to flow away from convex areas of the substrate, resulting in localized thinning during solder dipping or reflowing that can lead to loss of solderability of the part during storage. Degradation of solderability is believed to be caused by a combination of tin depletion in the thinned coating, which becomes Pb-rich as tin reacts with the copper substrate to form intermetallic species, and inadequate coverage of the underlying Cu-Sn intermetallic layer, which is unsolderable when oxidized.
Thinning of solder coatings is especially pronounced at PWB plated through-hole (PTH) rims. This condition is commonly referred to as "weak knees." When weak knees become unwettable after storage of PWBs, flow of molten solder into and out of PTHs is inhibited to such an extent that good solder joints cannot be obtained within permissible wave soldering dwell times. In extreme cases, solderability is lost completely.
FIG. 1A illustrates that solder thinning at a plated through-hole knee of PWB 11 during reflow results in a greater radius of curvature for the solder surface. This results because the total area in contact with the vapor phase, and consequently the total solder surface energy, is thus minimized. The knee area is plated with a conductive substrate 12 (typically copper) and a solder coating 14, as shown by a dotted line. Note that the liquid-solid interfacial area is always that of the electrically conductive substrate 12, which remains geometrically constant during the reflow process. During reflow, solder coating 14 assumes a more spherical surface 16 that causes thinning at the knee. The solder liquid-gas surface tension provides the driving force for the thinning process. If the effects of substrate surface roughness and solder inhomogeneities are neglected, surface tension theoretically would continue to act until the solder thickness at the knee is of molecular dimensions. Decreasing the surface tension (or increasing the viscosity) of the solder would decrease the rate of thinning but would not affect the minimum thickness at equilibrium. Such kinetic control of the thinning process is of limited effect because all areas of a large part, particularly a PWB, do not attain thermal equilibrium rapidly or equally. As a result, the actual reflow time, and thus the minimum solder thickness, will vary from area to area of the PWB. Because thin, variable solder coatings result in poor quality connections, there is a need for an effective method of attaining uniform solder coatings on plated through-hole rims of copper printed wiring boards.
Similar to the thinning effect at plated through-hole knees, molten solder tends to bead up on isolated, electrically conductive surface pad 13 on printed wiring board 11. FIG. 1B illustrates that as-plated solder 14 is essentially flat on top of pad 13. During reflow of solder coating 14 on surface pad 13, however, the solder assumes a more spherical profile, or bead 17. Rounded bead 17 can cause difficulty in aligning a component lead wire prior to soldering the wire to pad 13, which is used to connect surface-mount components to PWB 11. This is a serious problem for thin, fine-pitched lead wires, which tend to slide off rounded bead 17. Because the trend in the electronics industry is to use fine-pitched leads for surface-mount components, there is also a need for an effective method of maintaining flat solder coatings on PWB surface pads.